1. Field of the Invention
The present invention relates to a method and an apparatus for producing dry preforms for composite materials that exhibit an improved ability to be shaped into a curved shape required in various structures, including those used in aeronautic and space applications, architectural structures, automobiles, ships and other applications in which mechanical strength is of significant importance.
Hereinafter, the term “dry preform” is used to mean a fiber-made structure that has not been impregnated with a resin so that it can be formed into a desired shape by various forming processes including resin transfer molding (RTM), resin film infusion (RFI) and vacuum assist resin transfer molding (VaRTM).
2. Description of the Related Art
Lightweightness and high strength are general requirements for structures used in aircrafts and spacecrafts, architectural structures, automobiles, ships and other applications in which significant mechanical strength is required.
Traditionally, structural materials of airplanes, in particular those used to form curved structures and structures with continuously varying thicknesses and widths, have been formed of metal materials to provide a mechanical strength required for the particular structure. A large number of parts of significant weight must be used in these structures to provide a necessary strength by metal materials, adding to the weight of the airplane. Not only does this impose a significant limit on the design of airplanes, but it also leads to a considerable increase in the fuel consumption during actual flights of an airplane. For these reasons, developing lightweight materials has been a key factor in designing and manufacturing airplanes as well as in minimizing the operational costs of airplanes.
Many of recent studies conducted on structural materials of airplanes are directed to the development of fiber-reinforced composite materials with a high specific strength. Such materials are generally produced by impregnating a proper fiber reinforcement, such as glass fiber, aramide fiber, and carbon fiber, with an epoxy resin or other polymer matrices. Many of these materials have actually been put to use. Composite materials are considered superior to metal materials because of their high specific strength and high specific rigidity. Their anisotropic property, a unique characteristic of fiber reinforced material, also permits a high degree of freedom in designing different structures. Furthermore, composite materials are suitable for making a large structural unit as a single piece and can thus permit a significant reduction in the number of parts and costs.
To manufacture these composite materials, a technique is known in which a fiber-reinforced material is obtained by placing a plurality of pipes along the direction of the thickness of the material in a pattern corresponding to a desired shape, stringing strands of fiber reinforcement from one pipe to another in a zigzag fashion, and replacing the pipes with fibers that extend along the direction of the thickness to hold the strands of fiber reinforcement together (See, for example, Japanese Patent Laid-Open Publication No. Sho 59-47464). This technique, however, is not efficient since the strands of reinforcement fiber in a zigzag fashion need to be strung one at a time, which takes a considerable amount of time. Replacing the pipes with strands of fiber reinforcement that extend across the thickness further adds to the time and costs required by the process.
Also, a technique is known for producing a fiber-reinforced curved material with a complex cross-sectional shape (See, for example, descriptions of U.S. Pat. No. 5,914,002 and accompanying drawings). In this technique, strands of fiber reinforcements are arranged along straight lines and no strands are aligned to the desired curve. The fiber-reinforced material produced in this manner does not have sufficient mechanical strength.
When it is desired to form a fiber reinforced body that has a complex cross-sectional shape, such as a ring or any other curved shape, part or all of which is formed by a curve, the hand lay-up process for instance is employed. In this process, a material called prepreg, which is a fiber reinforcement pre-impregnated with a resin, is cut into small pieces of sheets, which are overlaid on top of one another to form a body of desired shape (See, for example, Japanese Patent Laid-Open Publication No. Hei 07-081566). A drawback with this technique is that most of the process needs to be carried out manually: the process is too complicated to be automated. This technique involves too many steps and requires significant amounts of time. Also, materials are wasted in large quantity in this process. Thus, the overall productivity of the process remains low. In this process, reinforcing fibers are provided in the form of ring-shaped or curved short fragments. This not only results in an insufficient mechanical strength of the structural material obtained by the process, as compared to structural materials reinforced by continuous strands of fiber reinforcement, but it also leads to a significantly increased cost.
In another technique for producing a fiber-reinforced material, strands of fiber reinforcement are strung to form a desired fiber arrangement with predetermined width and thickness, and the resulting fiber arrangement is unified, for example, by stitching to form a desired fiber-reinforced material (See, for example, description of U.S. Pat. No. 5,809,805 and accompanying drawings). Essentially, this technique is designed to produce intermediate preform substrates, which are subsequently formed into the final shape. Thus, it is difficult to directly obtain desired shaped products using this technique alone since the intermediate substrates require additional processing, such as cutting and overlaying, before the desired final shape is achieved. This makes the process rather complicated and costly.
A technique, known as the automated tape lamination (ATL) or the fiber placement process, that also makes use of a prepreg material is in practical use in the production of large components that have curved surfaces, such as those used as skin materials of aircrafts and rockets (See, for example, description of U.S. Pat. No. 6,096,164 and accompanying drawings). One disadvantage of this technique is that it cannot be used to make shaped bodies with an I-, L-, T-, hat-shaped or other complex cross-sections.
Processes that utilize fiber-reinforced fabrics are also known. In these techniques, fabrics are formed into a disk, a helix or other desired shapes (See, for example, Japanese Patent Laid-Open Publication Nos. Sho 57-133242, Hei 10-217263, Hei 09-207236, 2001-073241, Hei 07-133548, and 2002-3280). Each of these techniques, however, requires an extra step to form fibers into a fabric and thus is not cost-efficient. Also, these techniques involve many processes such as cutting and overlaying fabrics and require a significant amount of effort in precisely controlling the number of fibers in different areas of a fabric. As a result, each of these techniques tends to be costly.
Fiber-reinforced fabrics that take advantage of strands of fiber reinforcement with a complex cross-sectional shape, such as I-, H-, or T-shaped cross-section, are also known (See, for example, Japanese Patent Laid-Open Publication No. Sho 57-133241). Being a fabric woven on a weaving machine, this material permits only a limited number of arrangements of strands of fiber reinforcement: it is difficult to arrange fibers in a desired area and in a desired orientation. In addition, strands of fiber reinforcement aligned in the fabric material are crimped, leading to the insufficient strength of the material.
Techniques are also known in which fiber structure of a fabric is altered to make a curved structure (See, for example, Japanese Patent Laid-Open Publication Nos. Sho 63-120153 and Hei 02-191742). Fibers are also crimped in the fabric material used in these techniques and, as a result, the strength of the material is reduced. The techniques also involve many steps. Thus, these techniques are not suitable for mass production and can only achieve limited shapes of the products.
A process is also known that makes use of a braided material, which consists of interlaced strands of fiber reinforcement, to make a composite material (See, for example, Japanese Patent Laid-Open Publication No. Hei 10-290851). Although a typical braided material is deformable since it consists of fibers that are directed in two intersecting directions and are interlaced with each other, the deformability of the material is lost when fibers aligned in other directions are included to restrict the movement of the interlaced fiber reinforcements. Furthermore, the additional fibers may increase the unfavorably oriented strands of fiber reinforcement. Accordingly, not only does the process permit a limited degree of freedom in designing shaped products, but the weight and the size of the products are also increased in the process. Fibers are crimped in any of fabrics, braids and knits, reducing the strength of the products and making the process just as costly as the process using fabric. In addition, the strength required for a particular design of the structural material may sometimes not be obtained. Each of these problems has contributed to the delay in application of composite materials to commercial aircrafts (See, for example, Japanese Patent Laid-Open Publication No. 2000-328392).